Study On The Removal Of Heavy Metals From Aqueous Solutions By Reduced Graphene Oxide-supported Nanoscale Zero-valent Iron Composites

Water pollution occurs due mainly to inorganic and organic contaminants,such as nutrients,heavy metals and persistent organic pollutants.Among these pollutants,heavy metals are one of the most important classes due to their wide sources,non-degradability and high toxicity.Lead(Pb)and cadmium(Cd)are dangerous heavy metals,which are used as industrial raw materials for manufacturing storage batteries,pigments,fuels,pesticides,photography materials and explosives.Thus,the treatment or remediation of Pb and Cd pollution is of pivotal importance.In this study,reduced graphene oxide-supported nanoscale zero-valent iron(nZVI/rGO)composites were prepared by chemical deposition method and were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),Raman spectroscopy,N2-sorption,X-ray photoelectron spectroscopy(XPS)and Fourier Transform infrared spectroscopy(FTIR).Operating parameters for the removal processes of Pb(?)and Cd(?),such as temperature,pH,initial concentration and contact time,were optimized using the response surface methodology(RSM)model and the artificial neural network hybridized with genetic algorithm(ANN-GA)model.The optimum number of neurons in the hidden layer is determined based on the minimum mean square error(MSE).Then,developed RSM and ANN-GA models are compared against each other with respect to the accuracy of the Pb(?)and Cd(?)removal efficiencies prediction to better select a mathematical tool for modeling and optimization of the removal process.In addition,the Pb(?)and Cd(?)removal by nZVI/rGO composites were quantitatively evaluated by using adsorption isotherms,such as Langmuir,Freundlich and Dubinin-Radushkevich(D-R)isotherms.The experimental data were also fitted to Van't Hoff equation and kinetic models.Finally,the mechanisms for the removal of Pb(?)and Cd(?)ions by the nZVI/rGO composites were investigated based on the X-ray photoelectron spectroscopy(XPS)analysis.Results showed that the coefficient of determination(R2 > 0.99)of the mathematical models for the Pb(?)and Cd(?)removal indicates a high correlation between the experimental and predicted values.The optimal removal conditions for these two heavy metals were temperature of 21.30 °C,pH of 5.00,initial concentration of 400.00 mg/L,contact time of 60.00 min for Pb(?)and temperature of 37.13°C,pH of 7.00,initial concentration of 20.00 mg/L,contact time of 30.00 min for Cd(?)in the present study.The Pb(?)and Cd(?)predicted removal efficiencies by RSM were 100.37% and 85.92%,and the results from the verification experiments for these were 97.25% and 80.36%.The prediction errors in optimum removal efficiencies by RSM for the Pb(?)and Cd(?)removal were 3.11% and 6.47%,respectively.The MSE values(0.000257 and 0.002461)were obtained by using 6hidden neurons and 4 hidden neurons for the modeling of the Pb(?)and Cd(?)removal.The R2 of ANN models for the Pb(?)and Cd(?)removal were 0.9998 and 0.9999.The maximum predicted Pb(?)and Cd(?)removal were obtained when the temperature,pH,initial concentration and contact time were 25.47 ?,5.00,410.16 mg/L,58.60 min and25.31 °C,6.48,20.16 mg/L,30.00 min,respectively.The predicted and experimental Pb(?)and Cd(?)removal efficiencies for ANN-GA models were 98.27% and 98.63%,81.50% and 82.38%,respectively.The prediction errors of ANN-GA models for the Pb(?)and Cd(?)removal were 0.37% and 1.08%,respectively.It was found that the average prediction error of RSM was higher than that of ANN-GA,which shows the superiority of ANN-GA.The results obtained in this study illustrated that ANN-GA can be considered as an effective tool to model and predict the removal efficiency with a satisfied accuracy.The removal data of Pb(?)and Cd(?)were well fitted to the Langmuir isotherm,and the calculated maximum adsorption capacity was910 mg/g and 47.84 mg/g.The removal processes of Pb(?)and Cd(?)ions could be completed within 50 min and 30 min,which were well described by the pseudo-second order kinetic model.In addition,the dominant mechanism for the removal of Pb(?)ions is based on the adsorption with a small amount of reduction by the nZVI/rGO composites,while the mechanism for the Cd(?)removal is based on the adsorption.